1. Field of the Invention
The present invention relates to a method of installing an offshore wind turbine comprising the steps of
securing at least a part of a wind turbine to a transport vessel by bringing lifting means located on the transport vessel into contact with the part and raising the part in a parallel direction relative to the longitudinal direction of the wind turbine using the lifting means;
moving the transport vessel from a loading site out to an installation site;
securing the transport vessel to at least a part of an offshore foundation which is located at the installation site, wherein the offshore foundation is configured to receive and hold the wind turbine; and
lowering the wind turbine onto an upper part of the offshore foundation using the lifting means.
The present invention also relates to a vessel for transporting and installing an offshore wind turbine, where the vessel comprises
a hull having a deck and at least one side surface;
a slot arranged in one of the side surfaces wherein at least one wind turbine may be positioned;
one or more holding means for securing the vessel to an offshore foundation located at an installation site.
2. Description of Related Art
When installing offshore wind turbines, there exists a need for transporting the wind turbine out to the installation site as well as ways to assemble the wind turbine either at the site or before transportation. First, a foundation has to be secured to the seabed at the installation site, and then, a transition piece or intermediate piece of the wind turbine is mounted to the foundation, before the remaining parts of the wind turbine can be installed. Depending on the design of the foundation, the transition/intermediate piece may form the upper part of the foundation.
Today, the different pieces of the wind turbine or the partly preassembled wind turbine are loaded onto an installation vessel which is then moved to the installation site. The installation vessel is then jacked up, lowered onto the seabed or maintained in its position using a mooring system. The wind turbine is then assembled using a crane on the installation vessel. Another way of installing an offshore wind turbine is to preassemble the entire wind turbine onshore, and then load it onto a barge and move the barge out to the installation site. A ship having a crane is then used to install the wind turbine at the installation site. All these installation methods require the use of an offshore crane which in turns increases the installation costs and the total installation time, since it is expensive to use such a crane and it takes time to erect the crane if it is compacted during transport. The use of an offshore crane furthermore requires relative came wind condition so that the individual pieces can be lifted and installed safely, since the tall crane is sensitive to wind impacts.
U.S. Patent Application Publication 2010/0316450 A1 discloses an installing method where a number of wind turbines are preassembled at the harbour and then loaded onto a barge in a vertical position. The barge is then towed out to the installation site by a towing vessel. The barge is then positioned relative to the installation site using a fendering and mooring spread system. A wind turbine is then skidded to an installation position on the barge and optionally adjusted in the vertical position by means of a jacking system relative to the foundation extending above the water surface. The barge is then lowered so that the lower section of the wind turbine is positioned on the upper part of the foundation by using ballasting tanks or tensioning means connected to anchors located on the seabed after which the barge is towed away from the foundation and then re-raised using the ballasting tanks or tensioning means. This method requires the use of a mooring system comprising at least three mooring points to keep the barge in position during the installation process. The wind turbine is installed by lowering and re-raising the entire barge which in turns increases the total installation time, since this is a time consuming process. The geometric metacentric height of the barge is furthermore raised, since the wind turbines are positioned on the desk, thus making the barge more sensitive to waves and wind impacts. Furthermore, a mating system is needed between the foundation and the wind turbine to absorb the impact loads during the load transfer, since the barge can be moved relative to the foundation due to the waves and wind impacts.
International Patent Application Publication WO 2012/066789 A discloses a similar method where the wind turbines are loaded onto one end of the desk of an installation ship using an onshore crane. The wind turbine is then moved along the desk to other end using clamping means located at the top of two parallel support structures located on the desk. The ship is then jacking up on the seabed before the wind turbine is moved into position over the foundation. The ship is then lowered so that the wind turbine is positioned on the foundation. The loading of the wind turbines takes time, as well as the jacking up and down of the ship which all add to the total installation time. The transport requires low speeds since the wind turbines are positioned on the desk, thus making them more sensitive to the wind and waves.
International Patent Application Publication WO 2011/103988 A1 discloses an installing method where the entire structure, including the offshore wind turbine and the foundation, are preassembled at a small platform near the harbour. A transport vessel is then moved into position over the foundation and secured to the foundation by means of rollers and pulleys. The entire structure is then lifted and sailed out to the installation site. Support poles are then extended out of the hull of the vessel, and the foundation is then filled with ballast via a second ship. The transport vessel is then lowed via ballast tanks until the support poles contact the seabed. The foundation, including the wind turbine, is then lowed into position, and the rollers and pulleys are then disconnected and raised back up. The transport vessel is then re-raised using the ballast tanks, and then sailed away. This method requires the wind turbine and the foundation to be preassembled at the harbour. This method only works well if the foundation is a gravity foundation, since the foundation is used to provide stability using transport and installation. The transport vessel needs to be placed on the seabed before the wind turbine can be installed, which in turns adds to the total installation time. Furthermore, the foundation needs to be filled with ballast before being lowered into position, which requires the use of a second ship.
International Patent Application Publication WO 2011/108938 A2 discloses an installing method where the offshore wind turbine and the transition piece are preassembled at the harbour and then skidded onto a pivotal support structure on the barge in a horizontal position. The wind turbine is then secured to the support structure using an actuation device extending towards the slot. The barge is then towed out to the installation site. The support structure is then pivoted around a pivot point into a slot formed in the barge so that the wind turbine and the foundation are positioned in a substantially vertical position. The transition piece and the wind turbine are then positioned over the foundation using a dynamic positioning system, and then lowered onto the upper part of the foundation using the actuation device. The actuation device is then released and the barge is then towed away. This method requires the wind turbine to be rotated into the vertical position before installing the wind turbine, thus in turns adding to the total installation time. This support structure requires the use of winches located on the front of the barge to pull the wind turbine and the transition piece away from the support structure so that the transition piece does not comes into contact with the support structure while being lowered into position.
International Patent Application Publication WO 2010/059489 A1 discloses an installation method where the wind turbine is pre-assembled before being transported out to the installation site using a catamaran. The catamaran comprising an upwards extending support structure and a slot in which the wind turbine is positioned. The catamaran is lowered by filling the ballast tanks with water and moved into position relative to the wind turbine. The wind turbine is then secured to the support structure using clamps arranged on the support structure. The catamaran is then raised by emptying the ballast tanks and moved out to the installation site. The wind turbine is positioned on the foundation by lowering the catamaran again. The raising and lowering of the catamaran is a slow process which adds to the total installation time. The transport can only be done at low wind speed where the wind turbine can be held in a stable position, as the wind turbine is transported in an elevated position above the water surface.